How light and sound give physicists a clearer picture of cancer

An image of prostate tumours made by an optoacoustic imaging machine. Blue areas show the most aggressive cancer cells. Red areas show the least aggressive cancer cells. Credit: Michal Tomaszewski.

Imaging machines have been a mainstay of hospitals for decades, helping diagnose cancer and plan treatment.

For example, CT and MRI scans tell doctors where cancers are, along with key features such as a tumour’s size and shape. But these scans can’t zoom in and work out what’s happening inside the tumour as it develops and grows within the body.

Dr Sarah Bohndiek and her team at our Cambridge Institute are trying to tackle this shortfall, not by looking at tumours with x-rays, radio waves or magnets, but by listening to them instead.

Studying mice with prostate cancer, the team has been gathering sound information given off by tumours in response to light.

And in new research, published in the journal Theranostics, the team offers a glimpse of how these sounds can be turned into pictures, potentially showing how aggressive a tumour is.

The optoacoustic imaging machine

Whilst some scans are great at telling doctors about a tumour’s size and its shape, they aren’t designed to collect key molecular details that may give information about how aggressive the tumour is.

Some experimental techniques based on MRI and PET scans are showingpromise in this area. But Bohndiek’s team has turned to a different approach, called optoacoustics, as a possible way to tell us what a tumour’s really like and how it works inside the body.

“The technology is called optoacoustics because there’s an optical aspect and a sound related aspect,” says Michal Tomaszewski, a PhD student in the lab who led the latest study.

“How the machine works is that cancer tissues in a mouse with prostate cancer are lit up by a laser.

“The energy of the laser is absorbed by the tissues, which heat up slightly.”

And it’s the heating of the tissues, says Tomaszewski, that creates sound vibrations which are recorded by an ultrasound detector.

This information is then turned into an image like the one below:

An image of prostate tumours in mice made by an optoacoustic imaging machine. Light blue areas show the most aggressive cells. Red areas show the least aggressive. Credit: Michal Tomaszewski.

How the tumour reacts with the light, and the vibrations this creates, depends on certain characteristics of the cells. The team are looking at are the structure of the blood vessels around the tumour and how much oxygen the tumour cells can access from these blood vessels.

The structure of blood vessels and the balance of nutrients cells use is unique to every tumour, so a different image is produced each time. And this will change as the tumour grows and develops.

Tomaszewski’s results show how changing the amount of oxygen a mouse breathes alters the composition of the final picture.

“That’s the whole premise of the paper, to be able to tell how changes in blood oxygen, which we can measure with the optoacoustic scanner, relate to tumour biology.”

A tumour’s ecosystem

Working out how much oxygen tumour cells grab is a good indicator of how aggressive they might be.

“We want to know what the blood vessels look like and how well they work to deliver the oxygen and nutrients into the tumour cells,” says Tomaszewski.

This is because if the amount of oxygen becomes low the cells enter a state called hypoxia. And if there are too many tumour cells and not enough oxygen the cells start to compete.

The most competitive tumour cells switch to survival mode, which gives them characteristics that let them grow in low oxygen. It can also make them resistant to chemotherapy drugs or radiotherapy.

Patients with hypoxic tumour cells often have a worse outcome. So finding out which tumours are starved of oxygen could be vital for doctors.

At the moment though there is no standardised way of matching up oxygen levels and how aggressive a tumour is.

One way of assessing the characteristics of a tumour is by taking a sample (biopsy) of it. But biopsies are invasive, and may not always be possible.

Bohndiek’s team believe their scanning tech may offer an non-invasive alternative for measuring oxygen levels.

Understanding optoacoustics

For the first time the team has shown that there is a connection between the oxygen changes measured by the scanner and how well the tumour blood vessels work.

Tomaszewski is now trying to make these pictures easier for doctors to interpret. In the future this could help them spot which patients have aggressive tumours, that are surviving on low oxygen, and so need more aggressive treatment.

“Now we can say what the change in oxygenation means for the tumour biology. That’s the information the clinician wants,” says Tomaszewski. “They don’t need all that background physics.”

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